In typically-developing children, initially slow, one-at-a-time word learning is followed by an apparent "explosion" in their lexicon. One estimate suggests that children are learning up to nine new words a day through slow and deliberate effort (e.g., McLean, 1993). Yet word learning is a foundational component of human development, necessary for advanced linguistic functions (e.g. syntactic operations) and contributing to complex representation skills (for instance, categorization; Bowerman, 1988). Yet word learning is a foundational component of human development, necessary for advanced linguistic ; Bowerman, 1998). To the extent that a cognitive disability interferes with cord learning, progress in these domains will be correspondingly limited. It is therefore essential to explore in detail the nature of lexical impairments in mental retardation. The existence of difficulties in lexical acquisition among individuals with severe mental retardation raises important questions. When rapid vocabulary expansion occurs, what linguistic/cognitive processes support that learning? When it does not occur, what skills are deficient or absent? A phenomenon called fast mapping may be of greatest potential relevance for rapid vocabulary expansion. Fast mapping refers to a "quick, initial, partial understanding of a 'new' word's meaning" derived from the context of word use )Rice, 1989, p.152). Fast mapping has been argued to facilitate the vocabulary expansion. This proposed role has received empirical confirmation in typical youngsters and children with Down Syndrome )Mervis & Bertrand, 1994, 1995). Yet evidence from children with atypical cognition/language has recently challenged the universality of this relation. My own studies (Wilkinson & Green, 1997) have added to preliminary evidence of selective deficits in fast mapping among individuals with severe mental retardation, Williams Syndrome, or specific language purpose of vocabulary expansion. These recent findings oblige a more systematic analysis of the precise role of the principle in learning outcomes in mental retardation. Is fast mapping at risk in individuals with severe mental retardation, potentially limiting their vocabulary expansion? I propose a five-year study of vocabulary expansion and delay, focusing specifically on rapid expansion of an extant, but limited, vocabulary. This study will implement methods developed by the applicant (Wilkinson & Green, 199) that for the first time enable systematic analysis of unresolved questions of lexical expansion in severe mental retardation.